The current practice of halftone rendering for electronic printing involves using a threshold matrix and comparing the threshold values in the matrix with the image data values. In electronic printing, particularly for multi-color printing, the threshold matrix is often rotated, or sampled in a rotated fashion, to obtain a rotated halftone screen. One reason rotated screens are used is to minimize hue shifts due to any mis-registration between separations. A side effect is the presence of rosette patterns formed by the overlapping dot structure. These patterns can be objectionable, especially in lower resolution (200 to 400 dot per inch ) printers. When using rotated screens, moire patterns can also result as a product of interference patterns created from the screen angle and the screen ruling combinations.
In some applications, shifted screens have been used. A shifted screen is where all four separations are generated using the same screen angle (typically 45 degrees) and a small offset is applied in different directions, to each of the separations. This approach simplifies the halftone process because the threshold matrix does not have to be rotated. One desirable consequence of using a shifted-screen is that there are no rosette or moire patterns.
The organization of the threshold values, in the matrix, may be a dither matrix, center-weighted halftone dot or any other pattern. The center-weighted dots are typically used in conventional and electronic printing.
The aforementioned techniques have a limitation in that there must be a threshold matrix for each different screen ruling and angle, or alternately a method for rotating and scaling the matrix to support various screen angles and rulings if they are desired. Also, there often is an implicit limitation to the different number of screen rulings and screen angles that can be supported due to the fact that the output frequency value from the writer divided by the desired screen ruling value is usually required to result in an integer value. Wherein the integer value would be used to define the size of the threshold matrix.
The limitation of this approach has been that the number of tone or gray levels is limited to the number of elements in the threshold matrix. For example, for a .times.4 threshold matrix the halftone image will have only 16 tone levels.
A number of U.S. Patents teach the use of multiple threshold matrices, and a variety of methods to select the different matrices, at different times, to do the comparison against the input image data value. See, for example, U.S. Pat. Nos. 4,449,150, 4,495,522, 4,700,235, and 4,905,294. Several of these patents discuss the nature of the contents of the matrices, others do not. U.S. Pat. No. 4,495,522 uses the actual density value of the image to modulate the selection of the multiple threshold matrices. U.S. Pat. Nos. 4,449,150 and 4,905,294 select one of the multiple threshold matrices for each input value, based on a pseudo-random number which is supplied as a seed value by a counter (triggered by the clock signal driving the input channel). The random selection of threshold matrices in the prior art has been used primarily for purposes of moire and noise suppression.